At least one widely available commercial product is called 'synthetic marcasite,' and perhaps two or more. The most prominent is dark gray 'synthetic marcasite' for cheap jewelry. Possibly different is the synthetic marcasite used in battery research as at Electrochemical Society Proceedings.
I had suppposed the latter has the chemical composition and crystal strucure of natural marcasite. But I find that people also talk about marcasite iron diselenide and ditelluride, FeSe2 and FeTe2, apparently just based on crystal structure. Does anyone here know more about this?
The jewelry product is dark gray, and very cheap at prices like a few cents per small round stone in lots of 1,000, which you can easily find on line. So I wonder if it is actually one of the things called hematine, which are various ersatz hematites, not matching the composition of hematite but mimicking the appearance. See for example https://en.wikipedia.org/wiki/Hematine. Does anyone here know?